1. Field of the Invention
The present invention relates to an organic electroluminescent element and the manufacturing method, wherein the organic electroluminescent element includes a polymer organic luminescent layer and wherein the organic electroluminescent element is used for an organic electroluminescent display panel and the like. More particularly, the present invention relates to an organic electroluminescent element of which an organic luminescent layer is formed by a printing method. In addition, the present invention relates to a method of making an organic electroluminescent element which has a high grade of display without any or significant amount of unevenness and defect.
2. Description of the Related Art
An organic electroluminescent element has an organic luminous layer comprising an organic luminescent material between two opposing electrodes. By means of applying electric current to the organic luminous layer between both electrodes, the organic luminous layer emits light. Organic luminescent layer thickness is important so that the organic luminous layer emits light efficiently. So, organic luminescent layer thickness has to be about 100 nm.
Low molecular materials and polymeric materials are examples of organic luminescent materials that can be used to form the organic luminous layer. Generally, as for low molecular materials, thin film can be formed by vacuum evaporation. A mask with minute patterns is used at this time, and patterns are formed. In a manufacturing method with the use of vacuum processing such as vacuum evaporation, upsizing of a substrate can diminish the accuracy of the patterning. In addition, throughput is poor because a layer is formed in vacuum.
Thus the following method has been tried recently. Polymer organic luminescent material is dissolved in a solvent, and ink is made. This ink is used, and thin film is formed by a wet coating method.
A layer structure of an organic luminescent medium layer when an organic luminescent medium layer including an organic luminescent layer is formed by wet coating using a coating solution of high polymer materials is described below. The two-layer construction where a hole transport layer and an organic luminescent layer are laminated on an anode side is generally used. As for the organic luminescent layer, it is necessary for the organic luminescent inks including organic luminescent materials of red (R) green (G) and blue (B) in a solvent to be applied independently in order to form a color panel. (Japanese Patent Laid-Open No. 2001-93668 Official Gazette)
On the other hand, a hole transport layer is generally applied on a whole area contributing to imaging of an organic electroluminescent display panel without patterning the hole transport layer. A hole transport layer is formed by coating methods such as a spin coat method and a die coat method. Generally a hole transport layer is a thin film having a thickness equal to or less than 100 nm. Therefore, an electric current can flow more easily in a depthwise direction of a hole transport layer than a transverse direction of a hole transport layer. Therefore, it is said that leakage current to the outside of a picture element is insignificant if patterning of electrode is performed.
On the other hand, the present inventors manufactured an organic electroluminescent element of passive matrix type as follows. The pattern-formed pixel electrodes which are anodes were formed on a glass substrate. A pattern-formed insulator layer was formed between pixel electrodes. A hole transport layer was entirely applied in a significant surface of an organic electroluminescent element. A pattern-formed organic luminescent layer was formed. A pattern-formed cathode layer was formed. In this organic electroluminescent element of passive matrix type, current flowing between pattern-formed electrodes leaked out. Therefore luminous efficiency fell. In addition, contrast was lowered. Thus, it is necessary to form a pattern-formed hole transport layer only on pixel electrodes. In addition, it is necessary not to form a hole transport layer on an insulator layer.
However, a hole transport material forming a hole transport layer consists of a polymer material such as (3,4-polyethylen dihydroxy thiophen) (PEDOT). Hole transport ink is made by dissolving this hole transport material in a solvent. In consideration of solubility of a polymer material and necessity of thin film formation, it is necessary for the concentration of hole transport ink to be low. For example, the concentration is around 2%. And partition walls are necessary to prevent ink from spreading when the low-viscosity hole transport ink of which concentration is around 2% is printed in the shape of the pattern. A high insulator layer should be formed between pixel electrodes, and it should be used as a partition wall. Hole transport ink is applied on pixel electrodes sectioned by an insulator layer. In this case, a printing process such as an ink jet method or relief printing can be used. In addition, photogravure process is unsuitable to print on the substrate which is not flat.
An ink jet method is the method how organic luminescent ink is discharged multiple times on a substrate from ink jet nozzles. A discharge jet is separated from a substrate. Therefore, the ink spreads only by its own weight between partition walls on a substrate. Thus, it is difficult to apply ink to a whole area surrounded by an insulator layer by an ink jet method. Especially, print omission is easy to occur in edges of the pixel electrodes.
On the other hand, a method of avoiding print omission in an ink jet method is designed. However it is necessary to apply ink sufficiently to an aperture to prevent print omission. In this case, it is necessary to prevent ink from overflowing a partition wall. Therefore, it is necessary to do water-repellent processing for a partition wall. Then the number of the processes increases. Even more particularly, as for the ink applied between water repellent partition walls, it is in heaping full condition. Therefore, film thickness uniformity in a picture element becomes bad. Therefore, there is a problem in an aspect of homogeneity and stability of a display panel.
On the other hand, projection parts of a relief printing plate push a substrate in relief printing. In addition, a relief printing plate is buried in space surrounded by partition walls. Therefore, the ink is expanded in a picture element surrounded with partition walls. Thus, print omission in edges of pixel electrodes is hard to occur. In addition, relief printing method is simple and easy, and throughput is good as compared to ink jet method. Thus, when a hole transport layer is formed using hole transport ink, relief printing is preferred. However, when a good organic electroluminescent display unit is made by forming a uniform hole transport layer without defect and unevenness by relief printing, appropriate conditions of application of ink change according to a kind of printed pattern. Therefore specifying printing condition is necessary every time when a printed pattern changes.